A concave lens has focal length of 15 cm. At what distance should an object from the lens be placed so that it forms an image at 10 cm from the lens ? Also, find the magnification of the lens.

Explanation

Focal length of concave lens (`OF_(1)`), f = −15 cm
Image distance, v = −10 cm
According to the lens formula,
`1/v-1/u=1/f`
`1/u=1/v-1/f`
`=-1/10-1/-15=(-1)/10+1/15=-5/150`
u = -30 cm
The negative value of u indicates that the object is placed 30 cm in front of the lens. This is shown in the following ray diagram.